THEIR ACTION AS LEVERS 



287 



this refers to the angle of the attachment of the tendon to the lever which has to 

 be moved, and not to the general direction, which is often changed by the passage 

 of the tendon or muscle over i)rojections of bone or under arches of ligament before 

 it reaches its insertion. 



The advantage of an obliciue insertion may also be illustrated by the action of 

 muscular fibres, which cross one another like the two i)arts of the letter X in their 

 passage between two parallel liones; e.g. in the external and internal intercostal 

 muscles connecting adjacent ribs, or in the external and internal oblique muscles of 

 the abdomen in their passage from the crest of the ilium to the last rib. If the 

 muscles connecting the parallel bones ran at right angles to them, they could only 

 by their contraction dindnish the intervening space by one-half; whereas, running 

 obliquely, they produce l)y their contraction a much greater approximation of the 

 bones. For let A B and C D represent two parallel bones, and E F a muscular fibre 

 running perpendicularly from one to the other. If its full contraction reduced the 

 length of the fibre by one-half, it would merely draw the point E to E'. But suppose 

 that two muscular fibres C E and D E converge at right angles to each other upon 

 the point E; the amount that they will have to contract to draw E to E' may be 

 readily found by describing the arc of a circle E' K or E' L about C or D as a centre, 

 and with C E' or D E' as a radius. E K and E L will represent the contraction 



Fig. 261. 



required; and it is evident that if these oblique fibres contract more strongly, they 

 can bring the bone A B into close apposition with C D. 



When muscles pass over more than one joint, their action is somewhat more 

 complicated. Usually other muscles are at the same time called into play, so 

 as to fix some of the joints, and enable the muscle to act solely upon others. 

 For example, when the muscles arising above the wrist are used to extend the 

 phalanges of the fingers, it will be found that some of the flexors of the carpus 

 contract so as to fix the wrist. Similarly, if the fingers are flexed in grasping an 

 object, it is easy to feel the tendons of the carpal extensors starting up into firm 

 contraction. 



Occasionally the passage of a muscle over two joints is used to ol^tain very swift 

 and vigorous action. For example, if the long head of the triceps, which extends from 

 the axillary border of the shoulder-lilade to the olecranon process, Mere replaced 

 by an inextensible ligament, wliicli should be tight with the arm down and the 

 elbow flexed, the elevation of the humerus by the deltoid muscle would necessitate 

 a corresponding extension of the elbow. Now put in the place of the inextensible 

 ligament an actively contracting muscle, and it is clear that the combination of this 

 elevation of the arm by the deltoid with contraction of the triceps will produce a 

 much more rapid extension of the elbow, such as is seen in throwing a spear or a 

 stone. 



